Starting with Release 10, Third Generation Partnership Project (3GPP) Long Term Evolution (LTE) networks support Carrier Aggregation (CA). CA is where multiple Component Carriers (CCs) are aggregated and used for uplink/downlink transmission. There are basically two types of CA, namely, intra-band CA where the CCs are in the same frequency band and inter-band CA where the CCs are in different frequency bands. For intra-band CA, the CCs may be contiguous (i.e., adjacent in frequency) or non-contiguous (i.e., separated in frequency but still within the same frequency band).
For a particular User Equipment device (UE), one of the CCs used for CA is a Primary Component Carrier (PCC), whereas the other CC(s) is a Secondary Component Carrier(s) (SCC(s)). The serving cell of the UE on the PCC is referred to as the Primary Cell (PCell) of the UE. Conversely, a serving cell of the UE on a SCC is referred to as a Secondary Cell (SCell) of the UE. The PCell is always activated for the UE. In contrast, once a cell is identified as an SCell for the UE, that cell is configured as an SCell for the UE. Once configured, the SCell may be either activated or deactivated. As an example, for downlink CA, the UE receives downlink transmissions from a configured and activated SCell but does not receive downlink transmission from a configured and deactivated SCell.
In a CA scenario where one or more SCell(s) is configured but deactivated, the UE is expected to perform measurements on the corresponding SCC. In 3GPP Release 10, it was specified that UEs should be able to make such measurements without configured measurement gaps, as CA capable UEs (herein referred to as CA capable UEs, CA enabled UEs, or CA UEs) have Radio Frequency (RF) capability to receive multiple carriers simultaneously.
Subsequently, it was identified that even though CA UEs have the capability to receive multiple carriers simultaneously, starting or stopping the reception on an inactive carrier (e.g., to make measurements) may cause interruption to the other active carrier(s) that are being received. Thus, to enable power efficient CA implementation by starting and stopping the SCell reception, interruptions are allowed in some scenarios for 3GPP Release 10, for example when the deactivated SCell measurement cycle is configured to be greater than or equal to some specified value such as 640 milliseconds (ms). Each SCC of the UE may be configured with any one of the pre-defined values of the SCell measurement cycles: 160 ms, 256 ms, 320 ms, 640 ms, and 1280 ms. The exact value is still under discussion, and subject to change. When interruptions are allowed, the maximum allowed packet loss due to the interruptions is specified, for example 0.5% missed ACK/NACK rate.
Typically, interruptions are caused in the UE by retuning the RF local oscillator(s) used for the receiver(s) for the serving cell(s). Such reconfiguration of the RF may be necessary to change the receiver bandwidth (often for intra-band CA) so that the serving frequency and the target frequency to be measured can be received by a single receiver chain. Changing the receiver bandwidth also implies that the center frequency to which the receiver is tuned needs to be changed. Another reason for reconfiguring local oscillators is to reduce the impact of spurious emissions on the UE transmitter from the receiver local oscillators. This typically may be done for inter-band CA using a single chip RF solution. Finally, some interruption may occur due to the local oscillator pulling phenomenon. All of these causes of interruption are strongly related to UE architecture and cannot easily be predicted by the serving enhanced or evolved Node B (eNB).
From a system perspective, the interruptions are undesirable, especially as the serving eNB is unaware of whether a given UE makes interruptions; and if the UE does make interruptions, the serving eNB is unaware of the exact instants and durations when the interruptions occur. So, for example, the eNB may schedule the UE during an interruption, there may be impact to Outer Loop Link Adaptation (OLLA) especially if the Block Error Rate (BLER) target is close to or less than the packet loss rate, and so on.
As such, there is a need for systems and methods for avoiding interruptions on the serving cells of the UE resulting from reconfiguration of the UE for measurements on a SCC of a configured and deactivated SCell of the UE.